Exploration on long-term acclimation of Pyropia haitanensis thalli to monochromatic lights based on physiological characteristics and transcriptome analysis

Long-term acclimation to monochromatic lights emerges in seaweeds. In this study, analysis based on physiological characteristics and transcriptome sequencing were employed to investigate the long-term acclimation of thalli of rhodophyte Pyropia haitanensis grown under white light (WL) and monochromatic blue light (BL), green light (GL), and red light (RL). The net photosynthesis of synchronically cultured thalli was highest under WL, while the net photosynthesis was significantly higher under GL and RL compared with BL, indicating the low utilization efficiency of BL and relatively high ones of GL and RL. Compared with WL, the PE/Chl. a ratio was significantly higher in BL-acclimated thalli and lower in both GL- and RL-acclimated thalli. The C/N ratio was decreased in BL-acclimated thalli and drastically increased in GL- and RL-acclimated thalli. Only a small amount of starch grains were observed in WL-acclimated thallus cells, and no starch grain was found in BL-acclimated thallus cells, whereas numerous starch grains were found in GL- and RL-acclimated thallus cells. These findings implied roles of pigments and allocation of carbon between carbohydrates and phycobiliproteins during monochromatic light acclimation. Transcriptome analysis showed that DEGs involving protein synthesis were mainly upregulated by BL, whereas DEGs involving energy-yielding carbohydrate catabolism were mainly downregulated by GL and RL. Besides, genes encoding light-harvesting complex, ferredoxin-NADP reductase, and key enzymes of carbon and nitrogen assimilation were upregulated by BL, yet downregulated by GL and RL. Those results indicated that P. haitanensis thalli could compensate for the low utilization efficiency of BL via the increase in phycoerythrin accumulation through upregulating enzymes acting on carbon and nitrogen assimilation, consequently activating downstream of protein synthesis. Mechanisms of photoacclimation under GL and RL might include the reduction in light energy absorption and photosynthetic electron transfer chain activity via downregulating genes encoding nitrogen assimilating enzymes, light-harvesting complexes and ferredoxin-NADP reductases, and the attenuation in accumulation and catabolism of photosynthates to avoid excessive energy-yielding that might cause a burden on metabolism.